The term “expandable beads (or pellets) based on vinyl aromatic polymers” as used in the present description and claims, means vinyl aromatic polymers in the form of granules, containing an expanding system and optionally other additives.
These expandable thermoplastic polymers in the form of granules are particularly used, after expansion and moulding, in the production of household appliances or other industrial equipment, in packaging and thermal insulation in the building industry, due to their thermo-insulating properties. Thermoplastic vinyl aromatic polymers such as polystyrene can be made expandable by incorporating an expandable agent in the polymeric matrix. Typical expanding agents for vinyl aromatic polymers include at least one liquid hydrocarbon containing from 3 to 7 carbon atoms, a halogenated hydrocarbon, carbon dioxide or water. The quantity of expanding agent usually ranges from 2 to 15% by weight. Expandable polymers are produced in general as beads or granules which, under the action of heat, supplied, for example, by steam, are first expanded until a desired density is reached and, after a certain aging period, are sintered in closed moulds to produce blocks or the desired final products.
The making of such expandable beads has already been described in EP 126459, US 2006 211780, US 2005 156344, U.S. Pat. No. 6,783,710 and WO 2008 141766.
The present invention relates to a process in which an expandable agent and optionally additives are incorporated into the vinyl aromatic polymer in the molten state and then said vinyl aromatic polymer comprising the expandable agent and optionally additives is extruded through the die plate and cut to get pellets. Generally the die plate is incorporated in a machinery called “granulator” or “pelletizer” comprising,
means to introduce the molten vinyl aromatic polymer comprising the expandable agent and optionally additives,
the die plate,
cutting means to make the pellets,
means to cool and recover the pellets, e.g., circulating water and
means to separate the pellets from the water.
WO 2008 141766 describes such a process for the continuous production of granules based on thermoplastic polymers comprising at least one expandable agent and, optionally, other polymers or additives, among which inorganic pigments insoluble in the Polymeric matrix, wherein a first main stream is prepared, in the molten state, and a second stream in the molten State, which englobes the additives and which is added to the first stream. The mixture is extruded through a die which is cooled by means of water jets from nozzles positioned behind the cutting blades.
US 2009 0108480 relates to a process and an apparatus for pelletizing polymer melts comprising blowing agent in a pelletizing chamber through which a liquid flows, its pressure being above the ambient pressure. Another term generally used for the pelletizing process is underwater pelletizing, since water is generally used as liquid flowing through the pelletizing chamber. By way of example, the underwater pelletizing process is used when pellets are produced from plastics comprising blowing agent. The elevated pressure in the pelletizing chamber ensures that the plastic does not expand during the pelletizing process With plastics comprising blowing agent, it is generally the case that blowing agents are present in the polymer melt from which the pellets are manufactured. In said prior art in a first step, the polymer melt is injected into the pelletizing chamber, in a second step the polymer melt is cut via a cutting apparatus into individual pellets and, in a third step, the pellets produced in the pelletizing process are discharged with the liquid from the pelletizing chamber and are then isolated from the liquid. This process also comprises at least one of the following steps: (a) comminution of agglomerates or of pellets which exceed a prescribed maximum size, in a comminuting machine downstream of the pelletizing chamber or in a comminuting unit downstream of the cutting apparatus, (b) depressurization of the liquid in a depressurizing machine, (c) depressurization of the liquid in a throttle apparatus, where there is, upstream of the throttle apparatus, a pressure-equalizing container, (d) isolation of the pellets from the liquid, without any prior depressurization of the liquid with the pellets which it comprises.
US 2009 0091054 relates to a device for producing pellets from a plastic melt by extrusion, comprising a perforated plate from which the plastic melt is extruded at a pressure above the ambient pressure; a process chamber into which the plastic melt is extruded; a chopping device for chopping strands of the plastic melt extruded from the perforated plate into individual granules, the process chamber being filled with a process fluid; and a pumping device which supplies the process fluid to the process chamber at a pressure above the ambient pressure, the pressure of the process fluid with the therein contained granules being reduced downstream of the process chamber. According to the invention, an energy converter is provided downstream of the process chamber, wherein said energy converter extracts at least some of the pressure energy from the process fluid with the therein contained granules, reduces the pressure of the process fluid with the therein contained granules and converts at least some of the extracted energy into a reusable form of energy. The invention further relates to a corresponding process for producing pellets from a plastic melt by extrusion and also to a corresponding application.
US 2005 0156344 describes a process for the preparation of expandable styrene polymers having a molecular weight Mw of greater than 170000 g/mol, which comprises conveying a blowing agent-containing styrene polymer melt having a temperature of at least 120° C. through a die plate with holes whose diameter at the die exit is at most 1.5 mm, and subsequently granulating the extrudate.
In the prior art nothing is mentioned about the start-up, the shut down and other unstable phases of the process. In such a process the die plate comprises a plurality of small holes having a diameter typically in the range 0.5 to 1.9 mm diameter. Adjusting the introduction of the expandable agent and the optional additives takes some time, during said time the recovered pellets are off specifications. Moreover such a die plate needs to be operated close to the nominal capacity and typically not less than 75%, preferably not less than 80% of said nominal capacity. Until this operating range is not reached the recovered pellets are off specifications.
As regards a die plate having large holes it can be operated in a broader range compared to a die plate having small holes but it is not used to produce expandable vinyl aromatic polymer pellets comprising the expandable agent and optionally additives. The obtained pellets contain the expandable agent and optionally additives but they cannot be expanded properly.
It has been discovered to start-up the production on a die plate having large holes typically in the range 2 to 5 mm diameter and then,                as soon as the flow rate is close to the nominal capacity of the die plate having the small holes and        provided the proportion of expandable agent and optional additives are in the specifications,        to switch to said die plate having the small holes.The pellets produced during the start-up with the die plate having the large holes are kept and further can be sold as such or recycled in the process e.g., by a side extruder and mixed with the molten vinyl aromatic polymer comprising the expandable agent and optionally additives.        
During normal operation troubles can happen in the introduction of the expanding agent and/or the optional additives or in any equipment or even an equipment needs maintenance. The stream of molten vinyl aromatic polymer is switched from the pelletizer(s) having small holes to one or more pelletizers having large holes. When the troubles are over, the stream of molten vinyl aromatic polymer comprising the expandable agent and optionally additives is switched from the pelletizers having large holes to the pelletizers having small holes. As explained above the pellets produced during the troubles with the die plate having the large holes are kept and further can be sold as such or recycled in the process e.g., by a side extruder and mixed with the molten vinyl aromatic polymer comprising the expandable agent and optionally additives.